"I didn't want to fall into my comfort zone too much," she told the Radio Times. "I was unsure about signing away another year. Leaving me terrified, and that's what made me want to do it."

The 23-year-old actress will next be seen in Channel 4's Labyrinth, and was also linked to a role in Captain America movie sequel The Winter Soldier.

"Being afraid and going into the unknown excites me, and what scared me more was to keep going [on Downton] and then one day discover it was all I could do and wish I had pushed myself more," she explained. "I'd prefer to fail and fall flat on my face."